A new tridentate pincer phosphine/N-heterocyclic carbene ligand: Palladium complexes, their structures, and catalytic activities

Hon Man Lee, Jing Yao Zeng, Ching-Han Hu, Ming Tsung Lee

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Abstract

A new imidazolium salt, 1,3-bis(2-diphenylphosphanylethyl)-3H-imidazol-1- ium chloride (2), for the phosphine/N-heterocyclic carbene-based pincer ligand, PCNHCP, and its palladium complexes were reported. The complex, [Pd-(PCNHCP)Cl]Cl (4), was prepared by the common route of silver carbene transfer reaction and a novel direct reaction between the ligand precursor, PCNHCP·HCl and PdCl2 without the need of a base. Metathesis reactions of 4 with AgBF4 in acetonitrile produced [Pd(PCNHCP)(CH3CN)](BF4)2 (5). The same reaction in the presence of excess pyridine gave [Pd(PC NHCP)(py)](BF4)2 (6). The X-ray structure determination on 4-6 revealed the chiral twisting of the central imidazole rings from the metal coordination plane. In solution, fast interconversion between left- and right-twisted forms occurs. The twisting reflects the weak π-accepting property of the central NHC in PCNHCP. The uneven extent of twisting among the three complexes further implies the low rotational barrier about the Pd-NHC bond. Related theoretical computations confirm the small rotational energy barrier about the Pd-NHC bond (ca. 4 kcal/mol). Catalytic applications of 4 and 5 have shown that the complexes are modest catalysts in Suzuki coupling. The complexes were active catalysts in Heck coupling reactions with the dicationic complex 5 being more effective than the monocationic complex 4.

Original languageEnglish
Pages (from-to)6822-6829
Number of pages8
JournalInorganic Chemistry
Volume43
Issue number21
DOIs
Publication statusPublished - 2004 Oct 18

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All Science Journal Classification (ASJC) codes

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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